1. Field of the Invention
The present invention relates to an image pick-up device and an image pick-up system for picking up a subject image, and more particularly to an image pick-up element having poly-silicon wiring arranged on a light-receiving portion thereof.
2. Description of Related Art
As a conventional image pick-up element for introducing incident light into a light-receiving portion efficiently for performing a photoelectric conversion, for example, as disclosed in Japanese Patent Application Laid-Open No. H8-321595, a structure including an inter-layer lens in addition to a microlens as shown in FIG. 11 has been proposed. An image pick-up element 101 of FIG. 11 includes a light-receiving portion 102 formed in a silicon wafer 110, a microlens 109 for collecting incident light, a high refractive index portion 108, which is provided at a lower layer of the microlens 109 and has a refractive index higher than that of the microlens 109, and a low refractive index portion 107, which is provided at a lower layer of the high refractive index portion 108 and has a refractive index lower than that of the high refractive index portion 108. An inter-layer lens to be convex toward the center of the light-receiving portion 102 is configured at an interface between the high refractive index portion 108 and the low refractive index portion 107. In the image pick-up element 101 having such a configuration, incident light is first refracted toward the center of the light-receiving portion 102 when the incident light enters the microlens 109, and then the incident light is refracted at the interface between the microlens 109 and the high refractive index portion 108. Furthermore, the incident light is collected by the inter-layer lens, and then enters the light-receiving portion 102. By the use of the inter-layer lens, the incident light which cannot be taken in only by the microlens 109 enters the light-receiving portion 102.
For further improving a light collection efficiency, for example, Japanese Patent Application Laid-Open No. H11-68074 proposes an image pick-up element using a well structure. The well structure is a structure including a dug portion dug in the shape of a well in the bottom portion (shown by a dotted line) of the inter-layer lens configured at the interface between a high refractive index portion 208 and a low refractive index portion 207, as shown in FIG. 12. By means of such a well structure 221, incident light is totally reflected on an inner wall of the dug portion, namely the interface between the high refractive index portion 208 and the low refractive index portion 207, and then the incident light enters a light-receiving portion 202. Consequently, the light collection efficiency is further improved.
Generally, poly-silicon wiring made of poly-silicon (polycrystal silicon) is arranged on the light-receiving portion of an image pick-up element for performing the switching of the light-receiving portion, the control of electric charges, and the like. An image pick-up element 301 of FIG. 13 includes poly-silicon wiring 305 arranged on a light-receiving portion 302.
The image pick-up element 301 includes the light-receiving portion 302 on a silicon wafer 310, a microlens 309 having an optical axis of the center line L1 passing through the center of the light-receiving portion 302 in an X-direction, a well structure 321 composed of a high refractive index portion 308 and a low refractive index portion 307, a tapered portion 322 for guiding incident light into the well tapered portion 322 for guiding incident light into the well structure 321, and the poly-silicon wiring 305 formed on the light-receiving portion 302. The poly-silicon wiring 305 is arranged to overlap on at least a part of the light-receiving portion 302, and is electrically connected with the light-receiving portion 302. The poly-silicon wiring 305 is positioned at the side of the well structure 321 lest the poly-silicon wiring 305 interferes with the potential well structure 321. Pieces of metal wiring 306a and 306b for supplying power source or for controlling electric charges generated by the light-receiving portion 302 by the photoelectric conversion of the incident light are positioned on both the sides of the well structure 321 to put the well structure 321 between the pieces of the metal wiring 306a and 306b above the poly-silicon wiring 305. The pieces of the metal wiring 306a and 306b are severally made of, for example, an aluminum material, and have a light blocking effect. Moreover, both the pieces of the metal wiring 306a and 306b have the same height from the light-receiving portion 2 (in a Z-direction), and are arranged in parallel with each other in a Y-direction.
FIG. 14 is a top view for illustrating a positional relation between the respective pieces of wiring formed in the image pick-up element 301. A letter L2 denotes a center line perpendicular to the center line L1 in FIG. 13 in the Y-direction. As shown in the drawing, the light-receiving portion 302 is formed in an area having a distance of a in the Y-direction and a distance of b in the X-direction (in an area of a×b), and the center in the X-direction is positioned on the center line L2. A distance c being the interval between the pieces of the metal wiring 306a and 306b is formed to be smaller than the distance b. Moreover, the shortest distance between the poly-silicon wiring 305 and the center line L2 is c/2 to coincide with the distance and 306b has the light blocking effect, the substantially functioning part of the light-receiving portion 302 is an area of a×c. The well structure 321 is formed in the area of a×c in order to meet with the substantially functioning part of the light-receiving portion 302.
It is confirmed as to the image pick-up element using the well structure that, when the frontage thereof is too small, it is impossible to take in sufficient incident light, and that the light collection efficiency thereof scarcely differs from that in the case where no well structure is provided, substantially. On the other hand, the image pick-up element has been desired to be miniaturized furthermore in recent years. A problem in which the size of the frontage of the well structure cannot be sufficiently secured when an image pick-up element is tried to be miniaturized has been generated.